Delayed Halide‐Rich Molecular Passivation of CsPbCl ₃ Perovskite Nanocrystals Enables Bright Violet Light‐Emitting Diodes

CsPbCl 3 perovskite nanocrystals (NCs) are promising violet emitters owing to their narrow emission and high color purity, but their low defect tolerance demands careful passivation to achieve high photoluminescence quantum yield (PLQY), and typically only for fresh CsPbCl 3 NCs. Here, we report a delayed dual‐passivation pathway in CsPbCl 3 NCs induced by the halide‐rich molecular reagent phosphorus oxychloride (POCl 3 ), which unexpectedly yields a strong time‐dependent PLQY enhancement instead of the rapid degradation usually observed. POCl 3 gradually decomposes into P‐ and Cl‐containing species, enabling a controlled release of excess halides that autonomously passivates halide vacancies in a self‐regulated manner. This dynamic self‐healing process boosts the PLQY of colloidal CsPbCl 3 NCs by over 40‐fold relative to pristine samples and sustains high violet emission efficiencies for more than 2 months of storage under ambient conditions. Spectroscopic measurements and calculations indicate that both liberated Cl − and in situ—formed phosphonic species passivate halide vacancies and Pb 2 + dangling bonds, suppressing mid‐gap defect states. The resulting self‐passivated NCs deliver a luminance of 409 cd m − 2 , the highest reported for CsPbCl 3 ‐based violet emitters. These results establish halide‐rich dual passivators such as POCl 3 as powerful tools for long‐term defect control in chloride perovskite NCs and for robust, bright violet‐LEDs.